This invention is concerned with the production of furfural from pentoses (C-5 sugars) using a continuous plug flow reactor operated under a combination of four special conditions. High furfural yields are obtained with this present invention when the concentration of the reactants are optimized before entry into the reactor and are reacted at unusually high temperatures and short residence times in a plug flow reactor.
Pentosans (C-5 carbohydrates), which when hydrolyzed yield C-5 sugars (pentoses), are the major constituent of biomass hemicellulose. Pentosans can comprise 20-40% of the total dry biomass weight. State-of-the-art biomass acid hydrolysis processing techniques which breakdown pentosans to pentoses can achieve pentose yields to ninety percent of theoretical. Although the pentose hydrolyzate (solution) obtained by such techniques cn be used for other purposes, it can be converted to furfural.
Sources of pentosans include agricultural wastes, such as corn stover, sugar cane bagasse, rice hulls, and corn husks, pulp wastes from paper processing plants, newspaper and wood, and other similar cellulose materials. Pentoses can also be produced as a by-product in conjunction with other chemicals. For example in a wood to ethanol acid hydrolysis process, the wood hemicellulose can first be hydrolyzed to free the pentoses. Further hydrolysis of the remaining cellulose can then be accomplished. Glucose, the sugar derived from cellulose, can then be utilized as feedstock in a number of conversion processes including ethanol fermentation. The use of a by-product pentose stream, such as in the last example, as the feedstock in furfural production help make the overall process economics of this invention more favorable.
Previous inventions have also been concerned with the production of furfural using pentoses and mineral acid catalysts. For example, U.S. Pat. No. 2,536,732 reacts pentose with a mineral acid catalysts in either a continuous or incremental manner in a closed reactor which may be stirred. The reaction temperature is between 100.degree. and 210.degree. C. Reaction times exceed 15 minutes. A suitable furfural solvent is added incrementally or continuously to the reactor in countercurrent fashion. The type of reactor and reactor feed conditions used in U.S. Pat. No. 2,536,732 are not clear, but it is obvious that the reactor is not a plug flow reactor. The aqueous furfural-solvent mixture produced in the reactor is subjected to distillation and the solvent is recycled back to the reactor. U.S. Pat. No. 2,559,607 utilizes 1.5 to 10 percent xylose solution and 1.5 to 5 percent mineral acid catalysts in a tank-type reactor. The reaction temperature and average residence time are 140.degree.-165.degree. C. and 4-10 minutes respectively. The furfural is then extracted with a solvent, for example toluene, in a countercurrent extractor. The remaining aqueous pentose phase can be recycled to the reactor. These two patents use reactors wherein the pentose-acid residence times are distributed about an average lengthy resident time. This requires relatively large volume reactors. If continuous flow reactors of this type are used, reaction advantages based on higher initial concentrations are lost. These and other disadvantages make it difficult to obtain optimum yields and optimum reactor conditions in an efficient manner. The long resident times lead to adverse side reaction conditions. For example, in U.S. Pat. No. 2,559,607 in order to avoid side reactions, the furfural in the conversion zone must be kept below 0.7%. It is the purpose of this invention to provide a combination of four conditions for use with a plug flow reactor whereby the furfural yield is optimized.
Recovery of the product furfural from an aqueous stream can be achieved in a number of manners. Two common approaches being distillation of the furfural rich aqueous phase and solvent extraction of furfural from the aqueous phase with subsequent distillation of the furfural-solvent mixture to obtain the product furfural. U.S. Pat. Nos. 2,536,732 and 2,559,607 illustrate both of these methods. It is another purpose of this invention to provide a plug flow reactor process for producing furfural and recovery techniques to be used in combination with the plug flow reactor.